פירוש על קידושין 134:15

The analogy between a married woman and a wife’s sister is also difficult. The wife’s sister is prohibited as long as the wife is alive. It does not change at divorce. But a married woman is prohibited only while married. Once she is divorced, she is permitted. So maybe kiddushin is possible with a married woman but not with a wife’s sister. I know, this sounds absurd (that a man could betroth an already married woman), but sometimes arguments are made out of the necessities of logic and not because they make real sense.
In tomorrow’s exciting conclusion to this week’s daf we will finally learn how we know that betrothal with incestuous relations is not possible.